The present invention relates to a high sensitivity television camera and, more particularly, to a high sensitivity television camera utilizing a storage time control system.
An example of a construction of a conventional CCD (charge coupled device) television camera is shown in FIG. 5. The CCD television camera shown in FIG. 5 is constituted with a CCD pick-up element 51 for converting an image into an electric video signal, an image signal processing circuit 52 for processing the video signal input from the CCD pick-up element 51 in various manner, a CCD drive circuit 53 for controlling a pick-up operation of the CCD pick-up element 51 and a sync signal generator circuit 54 for generating a sync signal for synchronizing operations therein.
FIGS. 6 to 8 show an operation of the CCD television camera shown in FIG. 5, particularly, shows a so-called field storage mode operation. Signal charge stored in photo diodes 55 and 56 shown in FIG. 7 during a field period T21 (1/60 seconds) shown in FIG. 6 is read out to a vertical CCD 58 by a signal charge read pulse 59 at a time instance t21 and then added in the same CCD. The resultant signal charge is shown by a black circle .circle-solid. in FIG. 7. This signal charge is transferred through the vertical CCD 58 and a horizontal CCD (not shown) sequentially in the known manner and then is output externally as a video signal 61 of an odd number field.
Similarly, signal charge stored in photo diodes 56 and 57 shown in FIG. 7 during a field period T22 (1/60 seconds) is read out to the vertical CCD 58 by a signal charge read pulse 60 at a time instance t22 and then added in the same CCD. The resultant signal charge is shown by a black triangle .tangle-solidup. in FIG. 7. This signal charge is transferred through the vertical CCD 58 and the horizontal CCD (not shown) sequentially in the known manner and then is output externally as a video signal 62 of an even number field.
According to the operation in the field storage mode mentioned above, the adding direction of pixels in the odd number field is offset vertically from that in the even number field by 1 line, as shown in FIG. 8. For this reason, in a case of a CCD of NTSC system in which, for example, 490 pixels are arranged vertically, the vertical resoltuion becomes in the order of 350 TV.
FIG. 9 shows a construction of a conventional high sensitivity CCD television camera utilizing storage time control system, which comprises, in addition to components depicted by the same reference numerals as those shown in FIG. 5, a CCD drive circuit 63 having a storage time control function, an A/D converter 64, signal switches 65 and 66, field memories 67 and 68, a memory control circuit 69 and a D/A converter 70.
FIGS. 10 to 12 show an operation of the high sensitivity CCD television camera shown in FIG. 9 for a case where the sensitivity is twice the case of the normal drive shown in FIGS. 5 and 6 to 8, that is, where the charge storage time is 1/30 seconds. In FIG. 10, signal charge stored in photo diodes 71 and 72 shown in FIG. 11 during a charge storage time T23 (1/30 seconds) is read out to a vertical CCD 73 by a signal charge read pulse 74 at a time instance t23 every 1/30 seconds and then added in the same CCD 73. The resultant signal charge is shown by a black circle .circle-solid. and black triangle .tangle-solidup. in FIG. 11.
This signal charge is transferred through the vertical CCD 73 and a horizontal CCD (not shown) sequentially in the known manner and then is output as an analog video signal 75 from the CCD pick-up element 51 intermittently at a rate of once per frame. The analog video signal 75 is converted into a digital video signal by the A/D converter 64 and the resultant digital video signal is written in the field memories 67 and 68 alternately through the signal switch 65.
In order to convert the intermittent signal into a continuous signal, the digital video signal 76 stored in the field memory 67 is output through the signal switch 66 and the D/A converter 70 under control of the memory control circuit 69 as an analog video signal 77 which is the same in both the odd number and even number fields.
Japanese Patent Application Laid-open No. S64-78081 (1989) discloses a technique belonging to the technical field of the present invention. In the above-mentioned article, a control circuit which reads out electric charge from a solid state image pick-up element to be output every an integer multiple of field or frame period is used so that electric charge is read out in a high sensitive mode.
In the high sensitivity CCD television camera of the conventional storage time control system, however, the vertical resolution is extremely degraded since the same video signal is output in odd number field and even number field, although the sensitivity itself is improved proportionally to the storage time. That is, as shown in FIG. 12, the pixel sum in the vertical direction is fixed for odd number field and even number field. Therefore, in the CCD of the NTSC system in which 490 pixels, for example, are arranged in vertical direction, the vertical resolution is degraded to as low as about 245 TV.